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JOURNAL OF APPLIEDAQUACULTURE Each experimental biofilter was part of a biofiltration system that included a rectangular sedimentation tank (35.5 1), a mechanical filter composed of polyester batting, and an 800-! fish tank (Figure 1). The biofiltration systems included either a gas/water mixing column to dissolve a mixture of oxygen and ozone ~n the fish tank effluent prior to physical filtration or an oxygenation port at the base of the biofilter to oxygenate the water prior to biofiltration. Ozone was generated from bottled oxygen by means of an ORECy 03W9-O azonator. All systems were 100% recycle, except for water added to replace that lost to evaporation and leakage. Water temper- ature was maintained at 20°C in all biofiltration systems. Six types of biofiltration systems were investigated in this study. Each biofiltration system employed a different type of biofilter me- dia and either pre-oxygenation or pre-ozonation of the water prior to biofiltration. The only exception was the gravel biofiltration sys- tem, which employed neither pre-oxygenatton nor pre-ozonation. The other types of biofiltration systems were gravel + oxygena- tion, gravel +ozonation, granular carbon + oxygenation (fluid- ized bed), granular carbon +ozonation (fluidized bed), and granu- lar carbon +ozonation (not fluidized). Initially, each biofiltration system was operated for a minimum of eight weeks after asix- to eight-week acclimation period. Fish iaads and feeding rates were varied during this period to assess the effects of different levels of waste production vn biofilter perfor- mance. Twa of the biofiltration systems were operated with several different flow rates to assess the influence of hydraulic loading rate (i.e., water flow rate per unit of biofilter cross-sectional area) on biofilter performance. The gravel + oxygenation biofilter was op- erated at hydraulic loading rates of 5, 11, 22, and 44 ml/em2/sec- ond, and the granular carbon +ozonation biofilter was operated at hydraulic loading rates of 11 and 44 ml/cmZ/second. The biofilters were operated for a minimum of 17 days at each hydraulic loading rate. In later studies, the carrying capacities of five systems were as- certained: gravel; gravel + oxygenation; grave! +ozonation; gran- ular carbon + oxygenation (fluidized bed); and granular carbon + 1. Use of trade names does not imply endorsement.